This invention relates to a cyclic redundancy check monitor for the microwave landing system beam steering unit and more particularly for checking the accuracy of a series of digital command signals sent to the azimuth or elevation antenna phase shifters for such microwave landing system.
The microwave landing system (MLS) is a new international standard non-visual approach and landing system for aircraft. It consists of ground equipment disposed at airports and comprised of an azimuth antenna which sweeps a fan-shaped antenna beam from a predetermined position to and fro through a predetermined segment of azimuth and an elevation antenna which sweeps a second fan-shaped antenna beam up and down through a predetermined segment of elevation angle from a fixed predetermined point. Distance measuring equipment is located at a third fixed predetermined point, all of such points being located usually in close proximity to one another within the aircraft landing area.
An aircraft carries airborne microwave landing system equipment which includes means for receiving the signals transmitted by the ground equipment. In addition to distance measuring equipment which determines the distance to a specified ground point by conventional methods, the airborne MLS equipment includes means to separately receive and decode to and fro or up and down scanning beams transmitted from the ground system to derive the azimuth and elevation angles of the aircraft with respect to the known ground points. By these means an aircraft within range of the ground MLS equipment can actually determine its position in three dimensions. The accuracy of the system is such as to admit the aircraft to maneuver with great safety during periods of reduced visibility.
Precise control of each ground MLS transmitted antenna beam position during its to and fro or up and down scan is maintained by a component of the system ground equipment known as a beam steering unit (BSU). Briefly, each fan-shaped beam transmitted by the ground equipment is radiated from an electronically steerable phased array antenna, an azimuth antenna for the azimuth beam and an elevation antenna for the elevation beam. The actual angular position of a beam in space is controlled in a known manner by the setting of phase shifters associated with the antenna. These phase shifters are preferably digitally controlled. The BSU thus generates a cyclic series or train of digital command words which are applied to the phase shifters to cause the beams to scan to and fro or up and down as appropriate.
Perhaps the most critical characteristic of the microwave landing system ground equipment is the high integrity required to satisfy the guidance needs of aircraft within the airport approach and landing zones, such aircraft being entirely dependent on microwave landing system guidance. The integrity required places great importance on internal monitoring of the control signals of all ground system functions so that fault conditions can be quickly detected and isolated.